Menthol, particularly, l-menthol, is conventionally a very important flavor or fragrance having a pleasant cooling sensation and is applied to a wide variety of uses. As a process for obtaining l-menthol, a process of optically resolving dl-menthol and an asymmetric synthesis process are known (Synthetic flavor, written by Motoichi Indo, The Chemical Daily Co., Ltd, pp. 106 to 114). In a production step for l-menthol by the asymmetric synthesis, l-isopulegol as a precursor is hydrogenated to obtain l-menthol. In a step of synthesizing the l-isopulegol, a selective ring closing reaction of d-citronellal is important.
As to the selective ring closing reaction of d-citronellal, various processes have long been known. As a highly selective reaction using an aluminum complex as a catalyst, a highly selective ring closing reaction using an aluminum complex having 2,6-diphenylphenoxy moiety as a catalyst has been recently found (Japanese Patent Application Laid-Open No. 2002-212121). Other than this, a ring closing reaction using an aluminum complex having a phenoxy moiety as a catalyst (WO 2006/069659, WO 2006/092433, DE 102005023953) and a ring closing reaction using an aluminum complex having a siloxy moiety as a catalyst (WO 2007/039342) have been reported. However, there are no reports on a selective ring closing reaction of only one of the optical isomers of racemic citronellal by using an optically active aluminum complex. On the other hand, there are many reports on an aluminum catalyst having a biphenol skeleton as an axis-asymmetric ligand and a diol ligand having an asymmetric carbon (U.S. Pat. No. 6,090,950, U.S. Pat. No. 6,166,260, Angew. Chem. Int. Ed, 2001, 40, 92-138, Synlett, 1998, 1291-1293, Tetrahedron: Asymmetry 1991, Vol. 2, No. 12, 1295-1304, CROATIA CHEMICA ACTA, 1996, 69, 459-484 and Russian Chemical Bulletin, 2000, 49, 460-465). However, a catalyst having a ratio of aluminum:biphenol:diol of 2:2:1 or 2:1:2 has not yet been reported.